Combobox issues

Hi,

I can't find the solution in the Documentation. I want to read out all Keys of a simple Combobox and want to set the first occured Key. Then trigger an external event. Here is my Code:

myFuncOnInit: function() {

           this.fillComboboxViaAjaxAndSetData();

           var keys = this.getView().byId("idCombobox").getItems();   // <-- always []

           //var keys = this.getView().byId("idCombobox").getKeys();  // <-- always []

           //var keys = this.getView().getModel("modelCombobox").getData(); //<-- Empty Object

           var myKey = keys[0];

           this.getView().byId("idCombobox").setSelectedKey(myKey);

           this.fillTableWithAjaxAndSetData(myKey);

},

// only for show case

fillComboboxViaAjaxAndSetData: function() {

              var oModelCombobox = new sap.ui.model.json.JSONModel(),

                aUrl = 'http://example.com/test.xsjs'

             ;

            this.getView().setModel(oModelCombobox, "modelCombobox");

            jQuery.ajax({

                url: aUrl,

                method: 'GET',

                dataType: 'json',

                contentType: "application/json",

                success: function(data) {

                    oModelCombobox.setData(data[0]);

                }

            });

},

//XML

<ComboBox id="idCombobox"

       items="{modelCombobox>/}"

       selectionChange="selectionChangeFunc">

            <core:Item key="{modelCombobox>ID}" text="{modelCombobox>ID} {modelCombobox>Text}"/>

</ComboBox>

keys are always [], but I don't know why? There is no trouble filling the combobox or table.